Sound recording system



Oct. 22, 1935. w, A MaCNAIR 2,018,374

SOUND RECORDING SYSTEM Filed July 28, 1954 4 Sheets-Sheet 1 k g) '-L N v I n 9 Q m k INVENTOR. R Q G 5 By MAM/I6 IVA/ a g THJLWVM ATTORNEY Oc'a. 22, 1935. MacNAlR 2,018,374

SOUND RECORDING SYSTEM Filed July 28, 1934 4 Sheets-Sheet 2.

WA MAC NAIR BY A T TOR/VEV Oct. 22, W A MaCNAIR SOUND RECORDING SYSTEM 7 Filed July 28, 1954 4 Sheets-Sheet 3 wm QR INVENTOR By W.A.MAC IVA/R ATTORMEV Oct. 22, 1935. w, A, MEQNAIR 2,018,374

SOUND RECORDING SYSTEM Filed July 28, 1934 4 SheetsShee'h 4 N lNl/EN TOR By WA MAC NAIR ATTORNEY Patented Oct. 22, 1935 UNITED STATES PATENT OFFICE SOUND RECORDING SYSTEM Application July 28,1934, Serial No. 737,347

4 Claims.

This invention relates to sound recording systems, particularly film sound recording systems in which the light impressed on the film is modulated in accordance with the quality characteristics of the sound, and the quantity of light impressed on the film is simultaneously varied in accordance with the volume characteristics of the sound.

The object of the invention is the provision of a system which will be more certain, and flexible in operation, and in which the recording devices can not be overloaded by a surge of current due to a sudden sound of large volume.

A feature of the invention is the provision of means for supplying rectified sound currents to vary the quantity of light impressed on the film.

Another feature of the invention is the provision of unilaterally conductive means associated with the rectifying means to limit the rectified sound currents which may be supplied to the recording device.

Sound recording systems are known in which the intensity of the recording light varies with the volume characteristics of the sound while the beam is simultaneously modulated in accordance with the frequency characteristics of the sound and others in which the intensity of the recording light is modulated in accordance with the frequency characteristics of the sound while the width or position of the recording beam varies with the volume characteristics of the sound. A system of this character is shown for example in U. S. Patent 1,855,196 patented April 26, 1932 to W. H. Ofienhauser, Jr. In such systems, the modulated currents corresponding to the frequency characteristic of the sound are supplied through a controlled amplifier to the recording device. Due to the control exercised over the gain of the amplifier, the modulated currents supplied to the recording device are of substantially constant amplitude. The gain of the amplifier may conveniently be controlled by means of a slowly varying potential obtained from a detector supplied with sound modulated currents. A second detector also supplied with sound modulated currents supplies a slowly varying current to control the intensity of the lamp inversely to the variation in the gain of the amplifier.

As the gain of the amplifier is varied by means of detected sound modulated currents, when the amplitude of the sound modulated currents changes, a certain time will elapse before the gain of the amplifier has been changed to correspond with the change in amplitude. When the recording system has been operating for an appreciable time on sound modulated currents of fairly large amplitude, the gain of the controlled amplifier will still be fairly large. If a modulated current of still larger amplitude is suddenly applied to the system, before the gain of the amplifier can be ,5 reduced, this current, highly amplified, will be impressed on the recording device and may overload the recording device. Also, when this fairly large current was being recorded, the recording lamp was at nearly full brilliancy. Thus, if a 10 modulated current of still larger amplitude is now applied to the system, the recording lamp also may be burnt out. To avoid these difficulties, in accordance with the present invention, unilaterally conductive devices are placed in par- 15 allel relation with the voltage supply from the control detectors. A source of biasing potential is associated with the control detectors in such manner that, when the control voltage exceeds a predetermined value, current will flow through 20 the unilaterally conductive devices and prevent the control voltage from further increasing.

The drawings show in diagrammatic form a film sound recording system incorporating the invention. 25

Fig. 1 shows the signal amplifier;

Fig. 2 shows the recording device and power pp y;

Fig. 3 shows the control circuit;

Fig. 4 shows the power supply for the recording 30 light;

Fig. 5 is a keysheet showing the arrangement of the other figures.

In Fig. 1, the microphone l and amplifier 2 form a source of signal currents. Any other con- 35 venient source of signal currents may be employed such as a plurality of microphones connected to a mixer circuit, a radio receiving set, a telephone circuit, or the signal currents reproduced from a record such as a phonograph record or a sound 40 film. The signal currents may be transmitted through a high pass filter 3, having a lower cutoff at say 40 cycles. This filter eliminates any low frequency noise which may accompany the signals and aids in stabilizing the circuit.

The signals are transmitted through the transformer 4 and impressed on the input circuit of the amplifiers 5 and 6, connected in opposed or push-pull relation. As described in greater detail hereinbelow, the amplifiers 5 and 6 are va- 50 riable gain amplifiers having a substantially constant output for the major variations in amplitude of the signals.

The output of the variable gain amplifiers 5 and 6 is transmitted through transformer 1 and device of any convenient type.

further amplified in the two stage resistance coupled amplifiers 8 and 9. An interstage equalizer formed of an inductor l8 and capacitor Il may be placed in the output circuit of the amplifier 8 to cause an overamplification of the higher frequencies of the signals to compensate for the expected degradation of the higher frequencies in recording. The equalizer may be short-circuited by the switch I2 when not required.

The output of the two stageamplifiers 8 and 9 is transmitted through transformer I3, and impressed on the input circuit of the amplifiers I4, I5 connected in opposed or push-pull relation. The output of the amplifiers I4, I5 is applied through the transformer IE to a recording For convenience of description, the recording device disclosed in Fig. 2 is of the type described in an article, The Principles of the Light Valve," by T. E. Shea,'W. Herriott, and W. R. Goehner published in the Journal of the Societyof Motion Picture Engineers vol. XVIII pages 697-730 June,

1932. .It will be apparent to those skilled in the 7 art that the present invention is not limited to the specific light valve disclosed, but is applicable to all similar recording devices. The present invention may also be used in conjunction with many types of recording systems, such as the systems shown in U. S. Patents 1,853,812, April 12, 1932 to C. W. Hewlett 1,854,159, April 12, 1932 'to L. .T. Robinson, 1,921,037 August 8, 1933 to K.

F. Morgan, 1,933,267 October 31, 1933 to C. R.

Keith, et a1. and many others.

The output from the transformer I6 is supplied to the stretched conducting ribbons I1, I8 Fig. 2 immersed in the steady magnetic field of the magnet I9. The magnetic field due to the signal currents flowing in the ribbons I1, |8 will react with the magnetic field of the magnet I9 and cause the ribbons I1, I8 to oscillate in accordance with the signal currents. Light from a light source 20, is focussed'by the lens system 2| through apertures pieced in the pole faces of the magnet I9. apertures isfocussed bythe lens 22 on a film 23 traversed in known manner past a slot in an opaque plate 24. 'I'he ribbons I1, I8 define the height of the'film 23which, at any instant, is

exposed to light from the source 28.

tor 33 control the bias applied to thecontrol electrode of the oscillator. While a particular form of oscillator has been disclosed, other known types 7 of oscillators may be used.

The oscillating potential developed across the resistor 21 is applied through the capacitor 34 a to the control electrode of a variable gain amplifier 35. As described in detail hereinbelow, the

'1 variation in the gain of the amplifier 35 is in the 70 opposite sense to the variation in the gain of the amplifiers 5, 6m Fig. 1.

The amplifier 35 is coupled to a second ampli- The light emerging from the the oscillator 25. The output of the amplifier 36 is supplied to the transformer 40, through a capacitor 4|. The output of the transformer 40, further amplified in the power amplifier formed a of the three amplifiers 42, 43, 44 in parallel re- 5 lation, is supplied to the transformer 45, through the capacitor 46.

Current from a battery 41 flows through an inductor 48 and a. resistor 49, through wire 50 to the source of light 20, Fig. 2 thence through wire 5| back to battery 41. This current is adjusted by resistor 49 so that, in the absence of signal currents, the light from the source 20 falling on the film 23 is small. If the record produced on the film 23 is of the conventional vari- 15 able density type, the light falling on the film 23 may be adjusted if desired to produce an exposure at the lower end of the region of correct exposure. The output from the transformer also flows through wires 50 and 5| and the source 20 20 and increases the intensity of the light from the source 20 in accordance with the power supplied from the transformer.

Power from a suitable alternating current source is supplied to the primary'winding of the Q transformer 52, Fig. 2. The output of the secondary windings 54 and 55, connected in parallel, heat the cathode of the rectifier 51. The winding 55 is center tapped for the positive connection of .the rectified current. If desired, a single 3 winding may replace the windings 54 and 55. The outer ends of the secondary windings 53 and 56 are connected to the anodes of the rectifier 51; the inner ends of these windings are connected together to the negative connection of 55' the rectified current. Full wave rectified current flows from the tap on winding 55, through the smoothing inductors 58, 58', and the potential dividing resistors 59, 60, 6|, 62, 63, 64, to the junction of windings 53 and '56, thence alternately from each anode to the cathode of the rectifier 51. Smoothing capacitors maybe connected across the resistors as shown. Positive potential is supplied by wire 66 to the anodes of the amplifiers 5 and 6, Fig. 1. A smaller posi- *5 tive potential is supplied by wire 61 to the screen grids of the amplifiers 5 and 6.

Positive potential is supplied by wire 68 to the cathode of amplifier 35, Fig. 4. The control electrode of amplifier 35 is connected through're- 5d sistor 69 wire 10, resistor 1|, Fig. 3, wire 12 to the junction of resistors 64 and 65, Fig. 2. The control electrode of amplifier 35 is thus steadily biased negatively by the potential difference developed across the resistors 63 and 64.

A negative potential is supplied by wire 13, through resistor 14, Fig. 3, wire 15, smoothing inductor 16, Fig. 1 and resistor 11, balancing re-. sistors 18 and 19 to the control electrodes of the amplifiers 5 and 6. The cathodes of the ampli-go fiers 5 and 6 are grounded, and the junction of the resistors 63 and 64 is grounded. 'Thus the control electrodes of the amplifiers 5 and 6 are steadily biased negatively by the potential difference developed across the resistors 64 and 65. 65

Signal currents from the output of the high pass filter 3, Fig. 1, are'supplied through wires 60 and 8| and amplifier 82, Fig. 3 to a'potentiometer 83 across the transformer 84. The output of transformer, suitably amplified in amplifier 76 85 is supplied to transformer 86. Theoutput of transformer 86 flows along wire 81 through the rectifier 88, wire 89, resistors and 1|, wire 12, resistor 65, Fig. 2 wire 13 resistors 14 and 9| back to transformer 86-. The rectifier 88 is shown as 18 a thermionic triode, with indirectly heated cathode. Rectification takes place between the cathode and control electrode, while the anode is connected to the cathode to stabilize the action. The heater is supplied with power from any suitable source such as the transformer 92. The rectified signal currents flowing in the resistor 1| will produce a potential difference which will decrease the biasing potential on the control electrode of the amplifier 35, Fig. 4 due to the steady potential difference across resistors 63 and 64. The rectified signal currents flowing in resistor 14 will produce a potential difference which will increase the biasing potential on the control electrode of the amplifiers 5 and 6, Fig. 1 due to the steady potential difierence across resistors 64 and 65.

When the signal currents are of small amplitude, the biasing potentia1 on the amplifiers 5 and 6 is small, thus the signal currents are highly amplified and produce substantially maximum amplitude of oscillation of the ribbons I1, I8 of the light valve. At the same time, the biasing potential on the amplifier 35 is quite large, thus the high frequency currents from the oscillator 25 are only slightly amplified by the amplifier 35 and the lamp 20 is operating at low intensity. The original or negative record will thus be of low density, that is, the transmission is large. When this original record is printed in the usual manner, the positive print will be correspondingly dark, that is the transmission is small. The amplitude of the reproduced signal currents is proportional to the product of the degree of modulation multiplied by the average transmission. As the modulation is large, approaching 100 per cent, and the average transmission is correspondingly small the recorded signal, currents are not distorted and the reproduced signal currents will be of correct amplitude or volume.

Now, when the signal currents increase in amplitude, the rectified current from the rectifier 88 increases, and increases the potential difference produced across the resistors H and 14. The increased potential difference across resistor 14 increases the biasing potential on the control electrode of the amplifiers 5 and 6 and decreases the amplification of the amplifiers 5 and 6 proportionally to the increase in amplitude of the signal currents. The increased signal currents thus still produce substantially maximum amplitude of oscillation of the ribbons I1, I8 of the light valve. At the same time, the increased potential diiference produced across the resistor 11 decreases the biasing potential on the control electrode of the amplifier 35, the high frequency currents are amplified to a greater degree, and the intensity of the light from the lamp 20 increases. The original or negative record will thus be of increasing density. Thus, when this original record is printed, the mean transmission of the positive print will be greater than for signal currents of small amplitude. Again, the amplitudes of the recorded signal currents are not distorted and the reproduced signal currents will be of correct amplitude or volume.

The capacitor 93 is bridged across the resistors 90 and H and will be charged by the rectified signal currents. The charge will tend to leak off through the resistors 90 and 1|. The capacitor 94 is similarly connected across the resistors 9| and 14. These capacitors affect the timing of the controlling circuit so that, when the signal currents increase in amplitude, the control circuit responds fairly rapidly, whilst when the signal currents decrease in amplitude, the control circuit restores rather slowly. The control circuit thus will not follow the low frequency variations of the signal currents but approximately follows the variations in the mean amplitude integrated over several cycles. changes in the amplification of the amplifiers 5 and 6 and the amplifier 35 must be equal in effect and take place within the same time. As the filament of the lamp 2.0 has appreciable thermal inertia, the control circuit must not operate 10 faster than the intensity of the light can be changed. Thus, if the system is recording signal currents of about maximum amplitude, the lamp 20 will be of maximum brilliance and the ribbons I1, I8 of the light valve oscillating at full amplitude. To modify the input wave applied tothe control circuit so that the light valve will be fully modulated for a wide range of inputs, two Thyrite resistors 95 and 96 are shunted across the output winding of the transformer 84, Fig. 3. These resistors have the property of a comparatively high resistance for low voltages, and a low resistance for voltages exceeding a certain critical value. The properties of Thyrite" resistors are given in detail in the General Electric Review, vol. 33, No. 2, February 1930 page 92 and No. 6 June 1930 page 350.

The modification of the wave shape afforded by the Thyrite resistors however, is not sufficient to prevent a sudden transient from overloading the valve or burning out the lamp 20. For further protection, a rectifier 91 is shunted across the resistor H and a similar rectifier 98 is shunted across the resistor 14.

The rectifier 91 is shown as a thermionic triode, with the anode and control electrode connected together to form a thermionic diode. Other forms of rectifiers may obviously be used. Power from a suitable alternating source is supplied through transformer 99 to the heater elements of the rectifiers 91 and 98. Power from the same or a similar source is sup-plied through transformer I00 to the full wave rectifier IOI, which may be of the known copper-copper oxide type. The output of the rectifier IN is smoothed by the inductors I02 and I02 and capacitor I03 and supplied to the variable resistor I04. The potential difierence developed across the resistor H by the signal currents rectified in the rectifier 88 is applied along wire I05 to the anode of the rectifier 91, thence along wire I06 to the wiper of the variable resistor I04, and from resistor I04 along wire I01 to the other end of resistor 1|. A part of the potential developed across resistor I04 is thus opposed to the potential developed across resistor II. So long as the potential developed across resistor 1! is less than the potential developed across the part of resistor I04, practically no current will flow in rectifier 91. However, when the potential developed across resistor II exceeds the potential developed across resistor I04 by a small amount, a comparatively large current will flow in the rectifier 91. The potential developed across resistor H cannot further increase due to the low resistance 95 path through the rectifier 91. Thus, large transients are prevented from affecting the control circuit. By adjusting the resistor I04 the maximum voltage which is permitted to develop across the resistor 1I may be changed. From the operation it is obvious that the rectifier 91 may be any asymmetrically conductive device having a large resistance in one direction of flow and a small resistance in the other direction of flow.

In a similar manner, power from a suitable The reciprocal 5 source is supplied to transformer I 08, rectified by rectifier I69, smoothed by inductors III) and III) and capacitor III and produces a difference of potential across the variable resistor H2. The potential developed across the resistor 14 is applied to the anode of the rectifier 98, thence from the cathode along wire II3 to the wiper of the-variable resistor II2, and along wire II4 to resistor 14. As before, a. part of the potential developed across the resistor H2 is opposed to the potential developed across the resistor 14. When the potential developed across resistor 14 exceeds 7 the value determined by the setting of the wiper on resistor IIZ, current will flow through the rectifier stand the potential developed across resistor 14 will not rise any further.

In Fig. 3A, a single rectifier H5 is connected across both the resistor H and the resistor 14.

As before, a biasing potential is supplied from the full-wave rectifier I I 6 to the variable resistor I I1. In this case, when the combined potential difference developed across the resistors 1 I and 14 in series exceeds the biasing potential determined by the setting of the slider on resistor II1, current will flow in the, rectifier H5 and the potential difierence will not rise further. The control shown in Fig. 3A is cheaper and simpler than the control shownin Fig. 3, but is not as flexible in adjustment. 7

In Fig. 2, power from an alternating source is supplied to transformer I I8, rectified in full wave rectifier lit and supplied to the potential divider formed by the resistors I20, I2I, I22 in series. Current fiows through wire I23, and shunt feed inductor I24 to the anode of amplifier 36, Fig. 4 thence from the cathode through biasing resistor I25, wires I36, I3I, 68, resistor 63, to the grounded and of resistor I22. Current flows along wire I25, resistor I26 and shunt feed inductor I21 to the anode of amplifier 65, Fig. 3, thence from the cathode through biasing resistor I32 and ground to the grounded end of resistor I22. The heater element of amplifier 85 is energized by power from a suitable alternating source through transformer I28. Power also fiows along wire I33, Fig. I, through the smoothing inductors I34, I35 and resistor I36 to the anode of amplifier 9, thence from the cathode through biasing resistor E3"? to ground and back to the grounded end of resistor I22. Current from the inductor I35 flows through resistor I38, inductor I0, resistor I33 to the anode of amplifier 8, thence from the cathode through biasing resistor I40 to ground and back to the grounded end of resistor I22. Resistor MI is the usual grid coupling resistor.

Resistors I42 and I 33 reduce the hum that might be caused by variations in the current supplied to the amplifiers. Resistor I44 imp-roves the frequency response characteristic of the transformer 1. Current from battery I45 flows through resistor I46 through the heater elements of amplifiers 8 and 9 in parallel, along wire I41, through the heater elements of amplifiers 5 and 6 and resistor I 33 all in parallel along wire I 49 back to battery I455. The battery I45 may be replaced by any other suitable source of power if desired.

Current also flows from wire I33 through wire I55, smoothing inductors I5I, I52, resistor I53, primary winding of transformer I6, anode-cathode circuit of amplifiers I4 andI5, biasing resistor I54 to ground and thence to the grounded end of resistor I22, Fig. 2. Resistor I55 reduces the hum that might be impressed on the control electrodes of amplifiers I4and I5. The heater elem'entsof amplifiers 'I4'and I5 are energized from transformer I56 connected to a suitable source of alternating current.

Current flows along wire I51 Fig. 2, through inductor 38, Fig. 4' to the anode of amplifier 35; 5 and along wire I 58 through resistors 21, 26 to the anode of oscillator 25. The combined currents fiow from the cathodes of amplifier35 and oscillator 25 along wires I59, I3I, and 68 through resistor 63, Fig. 2, to ground thence to the ground- 10 ed end of resistor I22, Fig. 2. Current also fiows 1 along wire I60, to the screen grid of amplifier 35, thence to the cathode and joins the combined anode-cathode currents of amplifier 35 and oscillator 25. 15

In Fig. 4, power from a suitable alternating supply is supplied through transformer I6I to the filaments of the amplifiers 43 and 44; through the transformer I62 to the filament of amplifier 42 and the heater element of amplifier i0 36; and through transformer I63 to the heater elements of oscillator 25 and amplifier 35 and the filament of the full wave rectifier I64; Power is also supplied through transformer I63 to the anodes oi the rectifier I64 thence alternately 25 to the filament of rectifier I64, through smoothing inductor I65, wire I66, shunt feed inductor I61 to the anodes of the amplifiers 42, 43, 44, through the amplifiers to their respective filaments, and transformer windings, thence respectively through biasing resistors I68, I69, I10 and wire IN to transformer I63. The control electrodes of amplifiers 42, 43, 44 are connected together andthrough transformer 40 to the resistors I68, I69, I16. I

What is claimed is:

1. In a recording system, a source of modulated currents, means for amplifying said currents including a controlled amplifier, a circuit for supplying the output of said amplifier to a 40 recording device, a detector energized by said modulated currents and having an element controlling the output of said controlled amplifier,

an asymmetrically conductive device in parallel relation with said element, and a source of po- 45 tential in serial relation with said asymmetrically conductive device and connected to oppose the potential diiference developed across said element.

2. In a recording system, a recording medium, 50 a recording lamp, a modulating device interposed between said lamp and said medium, a source of modulated currents, means for amplifying said currents including a controlled amplifier,

a circuit for supplying the output of said am- 55 plifier to said modulating device, means for controlling the output of said amplifier including a detector energized by modulated currents, means for controlling the light from said recording lamp including a second detector energized by'modulated currents and having an element controlling the light from said lamp, an asymmetrically conductive device in parallel relation with said element and a source of potential in serial relation with said asymmetrically conductive device and opposing the potential difference developed across said element.

3. In combination, a source of modulated cur- V rents, means for amplifying said currents in- 70 eluding a, controlled amplifier, a device actuated by the output of said amplifier, an impedance element in the input circuit of said amplifier, a second impedance element in serial relation with said first impedance element, a third impedance 75- element in serial relation with said second impedance element, a rectifier energized by modulated currents, in serial relation with all said impedance elements, an electrical storage device in parallel relation with all said impedance elements, an asymmetrically conductive device in parallel relation to said first and said second impedance elements, and a source of potential in serial relation with said asymmetrically conductive device and connected to oppose the potential difference developed across said first and said second impedance elements.

4. In combination, a source of modulated currents, means for amplifying said currents including a controlled amplifier, a device actuated by the output 01' said amplifier, an impedance element in the input circuit of said amplifier, a second impedance element in serial relation with said first impedance element, a rectifier energized by modulated currents in serial relation with both said impedance elements, an electrical storage device in parallel relation with both said impedance elements, an asymmetrically conductive device in parallel relation to said first impedance element, and a source of potential in serial relation with said asymmetrically con-.

ductive device and connected to oppose the potential difference developed across said first impedance element.

WALTER A. MACNAIR. 

